Conventionally, a stratified scavenging two-cycle having a fluid mixture flow passage (not shown) for supplying a fluid mixture is connected to a crankcase 1 and an air flow passage 2 for supplying air is connected to a scavenging flow passage 3, is known (as shown in FIGS. 3 and 4). The scavenging flow passage 3 is continuously formed by a communication portion 30 extending from a crank chamber 1a to a cylinder chamber 4a, and a scavenging direction adjustment portion 31 extends from the communication portion 30 toward a cylinder inner surface 4b side and opens at a scavenging port 3a to the cylinder inner surface 4b.
The communication portion 30 is provided with a check-valve 10 at an air port 2a at which the air flow passage 2 is opened. The check-valve 10 permits air flow to the scavenging flow passage 3 from the air flow passage 2, and blocks back-flow to the air flow passage 2 from the scavenging flow passage 3. The entire body of the scavenging flow passage 3 is formed at the crankcase 1 and a cylinder block 4. The air flow passage 2 is formed within an air supply block 20. The air supply block 20 is attached to the cylinder block 4 with bolts, for example.
Meanwhile, the crankcase 1 is provided with a crankshaft 5, and a piston 7 is coupled to the crankshaft 5 with a connecting rod 6 between them. The piston 7 is fitted in the cylinder inner surface 4b and freely moves along an axial direction of the cylinder inner surface 4b. Further, the cylinder block 4 is provided with a cylinder head 8, which is provided with an ignition plug 9.
The scavenging port 3a leading to the scavenging flow passage 3 and an exhaust port (not shown) for exhausting combustion gas are opened to the cylinder inner surface 4b.
In the stratified scavenging two-cycle engine configured as above, as the piston 7 ascends, the pressure inside the crank chamber 1a starts to drop, and the scavenging port 3a and the exhaust port are sequentially closed. As a result, the fluid mixture in the cylinder chamber 4a is compressed, and the fluid mixture supplied from the mixture flow passage is passed into the crank chamber 1a. In this situation, air also enters the crank chamber 1a through the scavenging flow passage 3 from the air flow passage 2.
When the piston 7 reaches an area in the vicinity an upper-most position, the fluid mixture in the cylinder chamber 4a is ignited by means of the ignition plug 9, and thereby the pressure inside the cylinder chamber 4a rises and the piston 7 is descended. When the piston 7 descends to a predetermined position, the exhaust port and the scavenging port 3a are sequentially opened. As a result of the exhaust port being opened, the combustion gas is exhausted from the exhaust port, thereby the pressure inside the cylinder chamber 4a abruptly drops. As a result of the scavenging port 3a being opened, the air accumulated in the scavenging flow passage 3 spurts into the cylinder chamber 4a from the scavenging port 3a, and the combustion gas staying in the cylinder chamber 4a is compulsorily discharged from the exhaust port by the air. Thereafter, the fluid mixture in the crank chamber 1a enters the cylinder chamber 4a through the scavenging flow passage 3 from the scavenging port 3a. Thus the scavenging operation is completed.
Again the piston 7 ascends, and the aforesaid cycle is repeated once more.
According to the stratified scavenging two-cycle engine configured as above, the inside of the cylinder chamber 4a can be scavenged first by air, and combustible gas can be prevented from being discharged as a result of the fluid mixture blowing through, therefore obtaining an advantage that the exhaust gas is cleaned.
In the aforesaid stratified scavenging two-cycle engine, however, the air supplying block 20 is fixed on the side surface of the cylinder block 4, therefore causing a disadvantage that the engine size becomes larger, compromising compactness.